1. Field of the Invention
The invention relates to a process for making laminates of liquid absorbent tissues with an intervening layer of particulate material of high liquid absorbency. Particularly, the invention concerns such a process suitable for use with long length tissues traveling in the direction of their length at speeds in excess of 200 linear feet per minute.
2. Background Information
In the manufacture of absorbent materials, particularly those for use in absorbing body fluids, it has become generally recognized as desirable to incorporate in the absorbent tissues usually employed particulate material of higher absorbency to bulk ratios than the tissues, thus increasing the absorbency to bulk ratios of the product. Numerous granular or powder absorbents are available for the purpose, such as set forth in European Patent Publication No. 0080382, published June 1, 1983. Of these, the so-called "super absorbent" particles are particularly desirable because of their high absorbency.
Various processes have been proposed for incorporating such particulate material into absorbent tissue products. In the aforesaid European Patent Publication they are introduced into a stream of melt blown polymeric microfibres which is then formed into a web in which the absorbent particles are distributed and held by adhesion of the fibers to the particles. This requires special manufacture of a particular type of absorbent web.
Other processes utilize conventional prefabricated absorbent tissues with an intermediate layer of absorbent particles. In one such process, disclosed in U.S. Pat. No. 4,260,443, the particle layer is deposited on a dry substrate tissue, an overlay tissue is applied, the assembly is embossed to provide separated raised areas, and the overlay tissue in these raised areas is then moistened with water applied by a roller rotating in the direction of travel of the assembly which wets the particles in the raised areas sufficiently to make them tacky and adhesively bond together the tissues and intermediate particle layer in these areas. The product of this process is spot bonded only and likely to delaminate in subsequent handling and use. The absorbent particles, being loose between bonded areas, are free to migrate and bunch to an undesirable extent. Manufacturing problems include the difficulty of embossing absorbent tissues without rupturing them and the lack of applied heat and pressure after moistening, which our experience indicates would be necessary to durability of the spot bonds.
Another prior process making use of the inherent tackiness of the absorbent particles when slightly moistened was practiced commercially by applicants' assignee. In this process, the absorbent particles were mixed with a small amount of moistening agent (a polyhydric alcohol) before being applied as a layer to a base tissue. An overlay tissue was applied over the layer and the assembly was subjected to heat and pressure to set the bonds of the particles to the tissues, the particles having been rendered tacky by the moistening liquid, and thus bind the assembly together. The product of this process proved commercially acceptable, being of high absorbency and low bulk, with the tissues and particles adequately bonded together. However, manufacturing difficulties were experienced, due mainly to the fact that the absorbent particles were rendered tacky by moistening before they were applied to the base tissue. This caused the particles to tend to clot and string when it was attempted to shower a uniform layer of them on the tissue. Using a rotary brush and coaxial semi-cylindrical screen to produce the layer forming particle shower, acceptable results could be obtained but only at tissue speeds of 200 or less linear feet per minute, substantially lower than was desired for manufacturing cost purposes. Additionally, it was frequently necessary to shut down the process to unblind the screen of clots and strings of the sticky particles. At best, the particle layer did not have as much uniformity as desired, which led to a somewhat irregular extent of bonding.